1. Field of the Invention
This invention relates to silica optical fiber preform manufacture.
2. Prior Art
A method of optical fiber manufacture by a process involving the use of a chemical vapor reaction to deposit material upon the bore of a silica tube, the collapsing of the bore of the coated tube to form an optical fiber preform, and subsequently the drawing of optical fiber from the preform is described for instance in a paper by P.W Black and J. Irvent entitled `Development of High Silica Optical Fibre Waveguide` appearing in the Proceedings of the Electro-Optics Conference of March 1974 at Brighton, and also in a paper by P.W. Black entitled `Fabrication of Optical Fiber Waveguides` appearing in `Electrical Communication` Volume 51 No. 1 (1976). Particularly in the production of large core high numerical aperture fiber by this method, difficulties are liable to be encountered on account of differences in thermal expansion between component parts of the coated substrate. In general dopants such as phosphorus used to modify the refractive index of silica modify also the thermal expansion coefficient of the doped material. This mismatch is liable to cause the deposited material to become detached from the bore of tube if certain criteria of doping concentration and thickness of deposit are exceeded. This limits the numerical aperture and core size that can be obtained by this dopant material in fiber of given diameter produced from substrate tubing of a given diameter and wall thickness. For a particular doping concentration there is a limiting thickness that can be safely deposited on the bore of the substrate tube. A greater doping concentration has a smaller limiting thickness. Thus, for a specified substrate tube, a given doping concentration, and hence a given numerical aperture, determines the maximum ratio of deposited material to substrate material in the completed fiber. There is also a problem of dimensional control that is encountered when the deposition is made onto the bore of tube whose internal and external diameters are liable to vary significantly from piece to piece. If identical deposition conditions are employed to form deposits on tubes of different sizes, then, in the completed fibers, either the overall fiber diameters can be matched, or the diameters of their deposited waveguide portions, but not both simultaneously. This presents considerable problems in making certain types of optically efficient simple permanent or demountable butt couplings between pairs of fibers. It is possible to measure the dimensions of a tube before commencing the deposition process and then to tailor the deposition conditions appropriately. In the case of simple step index fiber this may not involve too much difficulty, but clearly more work is involved in making the necessary computations for making suitable adjustments in the manufacture of graded index fiber whose refractive index grading is required to obey some particular specified power law. A similar situation exists in the case of manufacture of other complex forms of fibre waveguide, such as the W-guide.